It is still available AI Image Resizer & Photo Enlargement Software - ON1
Well, the current version is €73,07
Back then (2004) I used Photoshop CS3.
If I need stitching nowadays, I use Affinity Photo. Here’s a 22 shot stitch taken with a Nikon D200 on a Manfrotto panoramic head
That’s a 97Mpx image without resizing. Here’s a screenshot of a detail at 100%

